1. Field of the Invention
The present invention relates to an X-ray CT (computed tomography) apparatus and a control method of the X-ray CT apparatus for reconstructed image data based on projection data derived from scanning of an object using the X-ray through an electrocardiogram synchronous reconstruction method.
2. Description of the Related Art
An X-ray CT apparatus provides images as the information of an object based on the intensity of the X-ray which has transmitted the object, and plays an important role for various medical practices including diagnosis/treatment of the illness and the surgery planning.
Improvement in the time resolution of the image is one of important tasks for conducting the examination with respect to the high-rate motion using the X-ray CT apparatus, for example, the heart examination. The aforementioned task may be achieved mainly by the use of both a half reconstruction method and an electrocardiogram synchronous reconstruction method. In the methods, the half projection data set collected during rotation of the X-ray tube in the angular range of 180°+α (α: fan angle of the fan-beam X-ray) around the heartbeat phase as the center designated by the operator is extracted, and a full projection data set is generated in the angular range of 360° from the extracted half projection data set through the 2D filter using so-called parker 2D weighting factor map so as to reconstruct the image data from the full projection data set in the angular range of 360°. Note that the heartbeat phase represents the position (%) with respect to the irregular duration between R waves normalized from 0 to 100%.
In the imaging operation using the X-ray CT apparatus, the time required for the rotation at 360° or the time required for the rotation at (180°+α) for the half reconstruction is limited as the substantial time resolution from the aspect of the image reconstruction in principle. Deterioration in the image quality owing to the blur is inevitably caused by the heartbeat rate in the substantial time resolution. In most of the case, it is difficult to designate the optimum heartbeat phase, that is, it is difficult to designate the heartbeat phase with the least motion in the time width of the substantial time resolution around the heartbeat phase as the center.
Japanese Patent Application Publication (Laid-open: KOKAI) No. 2007-37782 discloses the technique for obtaining the motion amount between the heartbeat phases by adding the projection data for identifying the optimum heartbeat phase for the electrocardiogram synchronous reconstruction so as to determine the heartbeat phase with less motion based on the obtained motion amount.
As the X-ray CT apparatus, the cone-beam X-ray CT apparatus has been disclosed, which has the X-ray tube for generating the cone-beam and the oppositely disposed X-ray detector with the large plane (2D) in pairs to rotate around the object for collecting the projection data required for executing the 3D image reconstruction to be executed by the computer. In the cone-beam X-ray CT apparatus, the cone-beam artifact may interfere with the 3D image reconstruction. The FeldKamp method for reconstructing the X-ray CT image disclosed in a following Document 1 employs the exact solution type algorithm. In the aforementioned case, the precision of the reconstructed image is generally high. It is therefore employed for the image reconstruction in the multi-slice CT and the cone-beam CT with the increasing rows.
It is known that the cone-beam artifact occurs more frequently in the reconstructed image remotely located in the body axis direction besides the reconstruction surface as the rotating center at which the X-ray source rotates as disclosed in following Documents 2, 3, and 4.
Document 1: Feldkamp, L. A., Davis, L. C., Kress, J. W., “Practical cone-beam algorithm” J. Oct. Soc. Am. Al 612-619 (1984)
Document 2: Turbell, H, “Cone-beam reconstruction using filtered backprojection” Linkoping Studies in Science and Technology, Thesis (2001)
Document 3: Wang, G., Lin, T-H., Cheng, P-C., Shinozaki D. M., “A general cone-beam reconstruction algorithm” IEEE Trans. Med. Imaging 12 486-496 (1993)
Document 4: Zeng, G. L., Gullberg, G. T., “A cone-beam tomography algorithm for orthogonal circle-and-line orbit” Phys. Med. Biol. 37 563-577 (1992)
Additionally, Published Japanese translation of PCT international Publication (Laid-open: KOHYO) for patent applications No. 2007-512936, and Japanese Patent Application Publication No. 2000-157535 may be referred to as the related art.
In the heart examination using the generally employed cone-beam X-ray CT apparatus, all the data in a row direction (slicing direction) are accumulated to specify the heartbeat phase. In case of the wide cone angle for forming the detection surface of 40 mm or larger, those data are not sufficient to specify the optimum heartbeat phase. The X-ray detector with the wide detection surface formed of the X-ray detection elements of 128 rows, 160 rows or 320 rows has been developed. However, even if such detector with the array of 128-row, 160-row or 320-row is directly applied to the method for setting the optimum heartbeat phase which has been conducted using the X-ray detector with the 64-row array, it is still difficult to set the optimum heartbeat phase under the influence of the cone angle.